A 'Heat Pump' picture for the ENSO system

Abstract

Why was the 1997-98 El Niño so strong? Will El Niño become more energetic in response to global warming? Why does the present climate have strong interannual variability in the first place? This talk will present observational and modeling results that shed new light on these fundamental questions. The observational results come from an analysis of the heat balance of the tropical Pacific over the last two decades. These results suggest that the magnitude of El Niño warming should be proportional to the equatorial upper ocean heat content which is in turn proportional to the intensity of the equatorial surface heating. Numerical experiments with a coupled model in which the ocean component is a GCM and therefore explicitly calculates the heat budget of the entire upper ocean support this suggestion. To accommodate an increase in the equatorial surface heating, the ENSO system sucks more heat into the equatorial upper ocean with a stronger zonal SST contrast during La Niña, and then pushes the heat poleward with a stronger El Niño warming. In this heat-pump picture for ENSO, El Niño acts as a regulator of the equatorial upper ocean heat content. Its existence and strength in the present climate owes not only to coupled wave dynamics, but also to the strong surface heating over the equatorial Pacific, and the high heat content the surface heating creates.